RC Slab
RC Slab
(OP)
When designing a RC slab, it is typical to spec the same reinforcement top and bottom. When analyzing this section, do you assume the concrete contributes nothing (i.e. a neutral axis of 0) when analyzing the stress blocks as the compression steel would balance the tension steel? As such, the moment capacity would be the force in the steel multiplied by the level arm between the T and C steel?






RE: RC Slab
Dik
RE: RC Slab
The bottom steel is designed for the required positive moment and the top steel for the required negative moment at each end.
Concrete design in the US, as defined by ACI utilizes the reinforcement (tension resistance) and the concrete compression side (compression resistance) in tandem to resist the applied moment.
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RE: RC Slab
RE: RC Slab
But you cannot make the design assumption you are talking about as the compression face steel will probably not be at yield or anywhere near it. In fact it might be in tension in a lightly reinforced slab.
RE: RC Slab
With any corners... due to flexure you can get 'corner levers' where the slab corners want to 'lift up'.
Dik
RE: RC Slab
RE: RC Slab
Dik
RE: RC Slab
While not an efficient way to detail a slab in many cases, I think that this logic is correct from the perspective of flexural strength. Utilizing the steel only should yield a lower bound moment capacity. One needs to ensure that the rebar detailing supports this approach of course. Any bar that is being utilized in compression needs to:
1) be anchored appropriately.
2) be spliced appropriately.
3) be prevented from buckling.
The third point is one that I've always wondered about. Compressed rebar in a column requires ties in order to prevent it from buckling. Obviously, your slab is unlikely to have such ties. Of course, anecdotally, slab reinforcement winds up inadvertently in compression all the time and I've never heard of it buckling and popping out of the slab. My guess is that slab compression rebar doesn't see the same kind of strain levels that column rebar does.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: RC Slab
You final sentence is answered by my last sentence. The compression strain in compression face reinforcement in a normal slab is very low. It would normally be near the neutral axis.
RE: RC Slab
Whaaaaat! You did not post-tensioning it?
RE: RC Slab
Ever try to tension a 6' strand? Span is so short, with anchorage losses...you almost have no stressing at all
Dik
RE: RC Slab
That was a bit of Aussie humour on Ingenuity's part. Those two know a bit about PT.
RE: RC Slab
Dik
RE: RC Slab
Actually, it was initially designed as PT, but a combination of house builders not being used to it and perimeter reinforced core filled block walls and lots of sloping edges and being too far from a major city to make PT economical forced a change to RC. So the Concrete industry got to provide extra concrete instead. Neighbours think it is the local cyclone shelter.
PS humour not allowed on this site thank you. Please desist or I will have to red flag you, and myself.
RE: RC Slab
Dik
RE: RC Slab
I see it. Somehow, I just never connected those dots before. Thanks for that. In cases where yield strains would be more likely to materialize in flexural compression steel, are you then of the opinion that it needs to be restrained laterally like column bars?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: RC Slab
The column rules assume the steel is at yield in compression, so the axial force in the bars is the full yield.
The Australian code does not require lateral restraint to be considered if the axial stress is below half of the yield stress.
I would assume in most normal slabs this would be the case. A very deep heavily reinforced slab might be different, not not your normal every day office/retail/parking/residential slab.
RE: RC Slab
Makes a lot of sense... as long as you have more than 1/2" concrete cover, the rebar is restrained.
Dik